This invention relates generally to an electro-optical device, and more particularly to a liquid crystal display device which includes at least two liquid crystal cells with each cell having transparent electrodes disposed on the opposed substrates.
Liquid crystal display devices including two liquid crystal cells have been provided. The first liquid crystal cell has a pair of substrates with liquid crystal material filled in the space between two substrates. Transparent electrodes are deposited on the inner surface of each substrate such that the transparent electrodes face each other. By applying a voltage across opposed electrodes, a display is produced by the first liquid crystal cell. The second liquid crystal cell includes in the space between the pair of substrates the liquid crystal material. However, transparent electrodes are not included in the substrates of the second cell. Thus, the second liquid crystal cell cannot by itself produce a display. The first and second liquid crystal cells are disposed between the facing surfaces of a pair of polarizing plates. Ideally, a black and white display having a high contrast is produced by the device.
FIG. 18 illustrates an exploded perspective view of one such conventional liquid crystal display device 100 including two cells. A first liquid crystal cell 110 and a second liquid crystal cell 120 are positioned between a lower polarizing plate 101 and an upper polarized plate 102. First liquid crystal cell 110 includes a lower e substrate 111 having an electrode 111a on its upper interior surface and an upper electrode substrate 112 having an electrode 112a on its lower interior surface. A liquid crystal material 113 fills the space between lower electrode substrate 111 and upper electrode substrate 112. By applying a voltage between electrode 111a and electrode 112a across the liquid crystal material a display is produced. Second liquid crystal cell 120 includes a lower substrate 121 and an upper substrate 122 with a liquid crystal material 123 filling the space therebetween.
The angle between the axial direction of liquid crystal molecules in contact with lower electrode substrate 111 and the polarizing axis (absorbing axis) of lower polarizing plate 110 is generally about 20.degree. to 70.degree.. As incident linear polarizing light passes through lower polarizer 101 it becomes linearly polarized. The linear polarizing light changes to elliptically (oval) polarizing light as it passes through first liquid crystal cell 110. The elliptical characteristics of the polarizing light are based on the refractive index anisotropy .DELTA.n, thickness d and twist angle of the liquid crystal material of first liquid crystal cell 110. The waveforms of the elliptical polarizing light have different major axial directions. The elliptical polarizing light also has different eccentricities. Eccentricity is defined as follows: EQU e=(a.sup.2 -b.sup.2).sup.1/2 /a.ltoreq.1
wherein a=the major axis of the elliptical polarizing light and b=the minor axis of the elliptical polarizing light.
As the elliptical polarizing light passes through second liquid crystal cell 120, the major axial directions of wavelengths in the visible range of the elliptically polarizing light are redirected so as to be are substantially the same. The twist angle and the product.DELTA.n.multidot.d (.mu.m) of second axial (rubbing) directions of the liquid crystal molecules in contact with lower substrate 121 of second liquid crystal cell 120 and in contact with upper electrode substrate 112 of first liquid crystal cell 110 are predetermined and set. Consequently, eccentricities e of the elliptically polarizing light are about 1 with respect to the waveforms within the visible range produced by second liquid crystal cell 120. Linear polarized light produced by second liquid crystal cell 120 results. Depending on the polarizing (absorbing) axis of polarizer 102, linear polarizing light is emitted by device 100 as white light. When the twist angle or product of birefringence .DELTA.n and thickness d (.DELTA.n.times.d) of first liquid crystal cell 110 changes, the twist angle and .DELTA.n.times.d of second liquid crystal cell 120 must be changed to ensure that white light can be produced by device 100.
In one embodiment of liquid crystal display device 100 first liquid crystal cell 110 has .DELTA.n.multidot.d set to approximately 0.9 .mu.m and a twist angle to the left of approximately 200.degree. (wherein the twist direction of the liquid crystal molecules extends from upper substrate 112 to lower substrate 111). Second liquid crystal cell 120 has .DELTA.n.multidot.d equal to approximately 0.7 .mu.m and a twist angle to the right of approximately 150.degree. (wherein the twist direction of the liquid crystal molecules extends from upper substrate 122 to lower substrate 121). The angle between the polarizing (absorbing) axis of upper polarizing plate 102 and the axial (rubbing) direction of the liquid crystal molecules in contact with upper substrate 122 of second liquid crystal cell 120 is approximately 40.degree.. The angle between the axial (rubbing) direction of the liquid crystal molecules in contact with lower substrate 121 of second liquid crystal cell 120 and the axial (rubbing) direction of the liquid crystal molecules in contact with upper electrode substrate 112 of first liquid crystal cell 110 is approximately 90.degree.. The angle between the axial (rubbing) direction of liquid crystal molecules in contact with lower electrode substrate 111 of first liquid crystal cell 110 and the polarizing (absorbing) axis of lower polarizer 101 is approximately 50.degree..
FIG. 19 illustrates the transmittance spectrum for the ON/OFF states of liquid crystal display device 100 based on the above-noted parameters. Selective/non-selective voltages are applied based on a multiplex drive having a 1/100 duty. High and low transmittance ratios are defined as the OFF and ON states, respectively. A pair of curves I and II represents the OFF and ON states, respectively. The amount of light transmitted through device 100 is shown along the ordinate based on suitable arbitrary units (AU).
Conventional liquid crystal display devices such as device 100 generally provide acceptable black and white displays. However, the refractive (refrax) index anisotropies .DELTA.n of first liquid crystal cell 110 and second liquid crystal cell 120 change based on the ambient temperature. Thus, a colored display, such as green or red rather than white is produced by liquid crystal display device 100 at certain ambient temperatures. The color of the display is based on the properties of liquid crystal materials 113 and 123 of first liquid crystal cell 110 and second liquid crystal cell 120, respectively. A colored rather than a monochromatic black and white display is also produced by device 100 when thicknesses d of liquid crystal material 113 and liquid crystal material 123 are erroneously beyond their predetermined values.
Accordingly, it is desirable to provide an electro-optical device which produces a monochromatic black and white display and which is not adversely influenced by changes in the ambient temperature conditions or by thicknesses d of the liquid crystal material which exceed their predetermined values.